PHYSIOLOGY OF THE NERVOUS SYSTEM 319 



muscle. Among other proofs is that furnished by cold-blooded animals, 

 like the frog, whose heart remains active for quite a while after its re- 

 moval from the body. These automatic contractions, however, are 

 not sufficient to meet all the demands made upon the circulation. The 

 needs of the tissues for the constituents of the blood vary with their 

 activity, and it is therefore necessary to vary frequently the force and 

 rapidity of the heart's contractions. Such changes the heart itself is 

 unable to bring about. 



For the purpose of controlling the rate and force of its contractions, 

 the heart is connected with the central nervous system by two kinds of 

 fibers : 



a. Fibers that convey excitant impulses to the heart to quicken its 

 movements. 



b. Fibers that convey inhibitory impulses to the heart to retard its 

 movements. 



The cell-bodies of the excitant fibers are found in the sympathetic 

 ganglia, but fibers from the bulb connect with and control them. The 

 cell-bodies of the inhibitory fibers are located in tlie bulb, from where 

 their fibers pass to the heart as a part of the vagus nerve. 



In addition to the fibers above mentioned, are those that convey 

 impulses from the heart to the bulb. These connect with neurons that 

 in turn connect with blood vessels and with them act reflexively, when 

 the heart is likely to be overstrained, to cause a dilation of the blood 

 vessels. This lessens the pressure which the heart must exert to empty 

 itself of blood. These fibers serve, in this way, as a kind of safety 

 valve for the heart. 



2. Control of Arteries. Changes in the rate and force of the 

 heart's contractions can be made to correspond only to the general 

 needs of the body. When the blood supply to a particular organ is to be 

 increased or diminished, this is accomplished through the muscular coat 

 in the arteries. The connection of the arterial muscle with the sym- 

 pathetic ganglia and the method by which they vary the flow of blood to 

 different organs has already been explained (pages 311 and 49), so that 

 only the location of the controlling neurons need be noted here. These, 

 like the controlling neurons of the heart, have their cell-bodies in the 

 bulb. It thus appears that the entire control of the circulation is effected 

 in a reflex manner through the nerve centers in the bulb. These centers 

 are stimulated by. conditions that relate to the movement of the blood 

 through the body. 



